Such processor may perform an auto or cross correlation function, or a structure function calculator. In an auto correlator a signal is divided into two channels. One channel signal is delayed and multiplied by the non-delayed channel signal. In a cross correlator a first signal is delayed and multiplied with a second non delayed signal. A structure function calculator determines the square of the difference between a delayed and a non delayed signal.
Digital signal processing enables highly accurate mathematical operations to be carried out on signals. Due to recent advances in logic speed complicated processing can be carried out in real time. Also advances in the statistical property theories of some events have simplified the processing of some functions. One example of this is in laser light scattering experiments, particularly in weakly scattering events.
Detailed investigation into the properties of light scattering led to the development of a single clipped digital correlator described in U.K. Patent Specification No. 1,290,336. This correlator allowed the processing of signals representing the arrival of single photons on a sensitive detector. From this a whole range of work has been made possible, for example, laser doppler velocimetry (LDV) where doppler shifts of laser light scattered off particles in a fluid can be processed to give particle velocity and turbulence.
In the above digital correlator a correlation function is accumulated from information obtained in successive sample interval channels. Increasing the number of sample channels allows further information to be obtained but results in increased equipment costs.